Deposition of ceramic thin films from aqueous solutions at low temperatures.

摘要

Ceramic thin films have received substantial interest and found numerous applications. A novel ceramic thin films deposition technique has been developed and studied systematically. This technique employs controlled hydrolysis of inorganic salt as precursor from aqueous solutions at low temperature. It starts with the nucleation followed by growth of the particles. The nucleation can take place through homogeneous or heterogeneous nucleation. One of the determining parameters seems to the degree of supersaturation, which is the thermodynamic driving force for nucleation. In light of this, our work first focused on the definition and calculation of degree of supersaturation, and our experiments then indicated the role of degree of supersaturation in nucleation and growth and further deposition processes. We found that homogeneous nucleation is prevalence when degree of supersaturation is high enough, while heterogeneous nucleation can play a more important role when degree of supersaturation is low enough. The growing film can avoid lattice strain between different microstructures. It seems that the preference toward either of the two mechanisms results in distinct film microstructures and morphologies. This diversity introduced two more research areas, one in modeling of intermolecular forces, and the other in film properties.;Specifically, we have studied the deposition of ZrO2, TiO 2, and SnO2 thin films on various substrates including bare Si and self-assembled monolayer (SAM)-coated Si substrates. We found that all the films can be deposited on both substrates. In addition, we found the TiO2 films display a large variety of morphologies, from very dense particulate structure to dendritic structure, due to different degrees of supersaturation. From classic nucleation theory and experiments, we determined the interfacial energy and critical nuclei diameter for nucleation of TiO 2 to be 0.072 J·m-2 and typically 0.5 nm, respectively. We developed a quantitative approach to employ the DLVO theory to explain deposition behaviors and provide theoretical guideline for material design. Dynamic nanoindentations suggested moduli of TiO2, ZrO2 and SnO2 films were 27.95, 21.72, and 11.94 GPa, respectively. Dye penetration test indicated great potential of TiO2 films in hermetic package. Refractive index of the as-deposited TiO2 films was 1.7824+/-0.0172 (at 590.44 nm) determined by ellipsometry.